In certain applications, titanium based alloys are utilized under conditions wherein there is frictional metal-to-metal contact between two titanium alloy surfaces. An exemplary use would be that between the frictional parts of the roots of the compressor blades against the discs of a jet engine. The blades are secured to their respective discs by means of a dovetail arrangement in which the dovetail base of a blade fits into a groove formed on the periphery of a rotating disc. During operation of the jet engine, centrifugal forces and vibration cause the blade to move in the groove. The fit of the dovetail base portion of the blade in the disc groove is designed to allow for this movement.
Local aerodynamic effects, resulting from the vane or strut positions relative to the rotating components in the compressor, will produce high frequency, low amplitude vibrations. The combination of high contact stress due to the centrifugal forces and high frequency vibration causes extensive surface damage to unprotected surfaces. Stated otherwise, maintenance of direct metal to metal contact between two titanium alloy surfaces leads to a high coefficient of friction with serious damage to both surfaces due to metal-to-metal wear of the metal alloy.
Generally, it is the fan and the front stage compressor blades operating under relatively low temperatures and which are manufactured from light-weight titanium based alloys which are prone to metal-to-metal wear, whereas the rear stage compressor components operating under relatively higher temperatures are formed from materials such as steel or nickel based alloys which are less prone to metal-to-metal wear.
The current commercial practice involves protecting the titanium alloy components of gas turbine and jet engines by the application of a thermal spray coating, preferably in conjunction with a solid film lubricant, to the dovetail base of the blade. The protective coating is composed of a soft copper-nickel alloy or a copper-nickel-indium alloy, which is applied by plasma spraying. Subsequently, the coating is dry film lubricated using molybdenum disulfide in an organic resin binder. This solid film lubricant initially provides a low coefficient of friction in an attempt to prevent galling, and to delay the onset of adhesive wear originating from metal-to-metal contact. This is described in ASM Handbook, Volume 18, October 1992, pp. 588-592 by Schell, J. D. et al.
Once this solid lubricant has been eroded, the CuNiIn coating on the dovetail base of the blade rubs against the titanium alloy disc groove surfaces damaging the disc by removal of the metal from the disc and by creating pits in the disc. The disc damage may lead to seizure of the blades within the disc grooves or, in extreme cases, to premature fatigue failure of the disc.
Thus, these prior art coatings have not proved fully successful in the prevention of the metal-to-metal wear of surfaces of titanium alloys in frictional contact with one another. Disadvantageously, too, such copper-nickel-indium and copper-nickel alloy systems display a good oxidation resistance up to about 315.degree. C. Usually, jet engines and the like are subjected to a broad temperature gradient in the fan/compressor which ranges from sub-zero (-60.degree. C.) up to approximately 600.degree. C. at the rear compressor stage, substantially above the effective 315.degree. C. upper temperature limit of the copper-nickel and copper-nickel-indium alloys.
It is well known in the art to utilize cobalt based alloys for overlay coatings on substrates to reduce oxidation and corrosion at elevated temperatures. In the U.S. Pat. No. 4,034,142 issued Jul. 5, 1977 to R. J. Hecht, there are disclosed overlay coatings for use exclusively in high temperature applications for the protection of substrates against oxidation and hot corrosion. The coatings contain aluminum, chromium, yttrium and silicon and a metal chosen from the group consisting of nickel, cobalt and iron or mixtures thereof. The coatings are particularly suited for the protection of nickel and cobalt superalloys at elevated temperatures, i.e. of the order of 1000.degree. C., by the formation of a stable oxide surface layer of alumina on the coatings which acts as a diffusion barrier to minimize further reactions.
A. R. Nicoll, in U.S. Pat. No. 4,503,122 issued Mar. 5, 1985, provides a high temperature protection layer for temperatures above 600.degree. C., usually substantially above 900.degree. C., for high temperature gas turbine parts manufactured from an austenitic material such as a nickel superalloy. The layer is composed of a base of chromium, aluminum and cobalt with silicon and yttrium. Again, the protective nature of the coatings relies on the formation of a continuous cover of an alumina skin resistant to high temperature corrosion at above 900.degree. C.
W. J. Brindley et al, U.S. Pat. No. 5,116,690 issued May 26, 1992 discloses overlay coatings of MCrAlX in which M may be nickel, cobalt or iron and X may be yttrium, Yb, Zr, or Hf on Ti.sub.3 Al+Nb titanium alloys in an oxidizing environment.
The overlay coatings disclosed in the Hecht, Nicoll and Brindley et al patents are used at temperatures in excess of 900.degree. C., well above the temperatures at which titanium alloys are used. The overlay coatings protect the airfoil of a blade against destructive influence of hot gas atmospheres. No protection for wear due to rubbing of an overlay coating against another solid surface is contemplated.
U.S. Pat. No. 4,789,441 issued Dec. 6, 1988 to J. Foster et al and U.S. Pat. No. 4,810,334 issued Mar. 7, 1989 to F. J. Honey et al disclose protection layers on turbine blades comprised of particles of chromium, aluminum, yttrium and silicon in a matrix of cobalt applied by composite electrolytic deposition. The protective layers and anchoring coats of a larger particle size are applied by electrolytic deposition and then spray coated with a thermal barrier of a refractory material by plasma deposition. These patents thus relate to the use of a cobalt alloy to anchor a ceramic coating to a substrate.
Privett III, et al. in U.S. Pat. No. 5,292,596 issued Mar. 8, 1994 provides a method for protecting a force-transmitting or force-receiving surface of titanium from fretting fatigue. The composition of the coating used is essentially, by weight, 30 to 70% cobalt, about 25 to 55% nickel and about 5 to about 25% iron. The essential feature of the patent disclosure is the presence of iron which, when oxidized to hematite at elevated temperatures, in the range of 480 to 650.degree. C. provides the improved anti-fretting properties.
Luthra et al. in U.S. Pat. No. 5,077,140 issued Dec. 31, 1991 disclose a method for protecting substrates from oxidation at temperatures of up to about 900.degree. C. The coatings consist of a continuous coating of ductile MCrAl or ductile MCr alloys where M is at least one metal selected from the group consisting of iron, nickel and cobalt.
Luthra et al.'s coatings protect titanium substrates against oxidation and not against metal-to-metal wear. According to Luthra et al., the coatings are useful for temperatures above 600.degree. C. when the titanium alloys have a high affinity for oxygen. Luthra et al. coatings do not provide metal-to-metal wear protection and, furthermore, they are intended only for elevated temperatures in a restricted range of 600-900.degree. C.
Cobalt alloy coatings useful as wear-resistant materials are exemplified in such products as "Stellite"* and "Tribaloy"*. The Stellite type coatings are typically composed of a chromium, tungsten strengthened cobalt matrix containing a high percentage of very hard carbides, predominantly chromium carbides. The Tribaloy type coatings are either cobalt or nickel based with molybdenum, silicon and chromium as the major alloying elements. The Tribaloy compositions are so balanced that the bulk of the structure is in hard, brittle, laves phases having a Rockwell Hardness (HRC) in the 50 to 60 range. Both Stellite and Tribaloy alloys are so hard as to prove unmachinable, and it is this hardness which is responsible for the wear resistant properties of coated articles. Unfortunately, should a softer part, such as one formed from titanium based alloy, be rubbed against by the coating-hardened surface, the latter will cause the softer part to wear excessively.
Cobalt based superalloys have been developed for their high temperature mechanical properties and oxidation resistance. They are typically used as wrought or cast solid parts, not as a coating. Although such alloys will not damage other superalloy surfaces when in contact with them, they cannot be used with titanium alloys without deleterious effects thereto.
*Trademarks